Stars are not usually purple, green, or pink. However, a recent study of white dwarf spectra has shown that, viewed up close, these compact stars could appear in precisely these colors. Why do they take on such unusual hues?
A Close-Up View of White Dwarfs
Astronomers at the University of Warwick have shown that white dwarf stars can shine in various colors, including the first identified star that would appear green to the human eye, according to phys.org.
For most stars, color indicates temperature. Cooler stars appear orange, those resembling our Sun appear white, and the hottest are pale blue—much like a blacksmith’s iron, which glows red, then orange, and finally white-hot as it heats up.
White dwarfs, the dense remnants formed when stars like the Sun reach the end of their lives, were thought to follow the same rule. They received their name because the first examples discovered were remarkably hot and glowed white or pale blue. But are orange, white, and blue the only colors in which a star can shine in the night sky?
According to the new University of Warwick study, the answer is no. Published in RAS Techniques and Instruments, the research shows that, if we could see them up close, some white dwarf stars would glow in shades of green, pink, turquoise, yellow, and purple.
“Most stars, including white dwarfs, behave like ‘ideal emitters,’ so their color is determined almost entirely by their temperature. But no one had really investigated what else might be possible for white dwarfs,” said Dr. Mark Hollands, the University of Warwick astronomer who led the work.
Why Do White Dwarfs’ Colors Shift?
The answer lies in density. The atmospheres of the coolest white dwarfs can reach approximately 1 gram per cubic centimeter—roughly the density of water and millions of times denser than the Sun’s photosphere.
Under these conditions, any traces of elements such as carbon, calcium, and sodium interact with the surrounding dense helium plasma and absorb light across a broader range of colors than they would at lower densities. This shifts the star’s color away from the orange, white, and blue hues of an ideal emitter.
No white dwarf is close enough for its color to be seen directly, so the team instead calculated it using data astronomers already routinely collect: the star’s spectrum, a “fingerprint” produced when its light is separated by wavelength, much as a prism splits sunlight into a rainbow.
By running these spectra through a model of human color vision, the scientists determined what color each star’s light would appear to the eye. This serves as a reminder of how much a spectrum can reveal—not only a star’s temperature or chemical composition, but also its true color.
A Star’s Chemistry Determines Its Hue
Applying this method to white dwarfs across the sky, the team found that their colors fall into distinct groups, each associated with the contents of the star’s atmosphere.
Carbon-rich white dwarfs tend to be pink or purple, while those enriched with debris from shattered asteroids and comets—the remnants of old planetary systems—lean toward yellow. Cooler white dwarfs with a mixture of hydrogen and helium in their atmospheres often take on a turquoise or pale-blue hue, caused by hydrogen molecules colliding with helium atoms in their ultradense atmospheres.
The most striking example is WD J1330+6435, which has a temperature of approximately 4,000°C (7,232°F) and has accumulated rocky material from its planetary system. This has enriched its atmosphere with calcium, lithium, and sodium, giving it a yellow-green appearance in the team’s models. To the researchers’ knowledge, no other star, living or dead, has ever been reliably described as green, making it the first known truly green star in the Galaxy.
At the opposite end of the spectrum, the team identified what may be the reddest star ever recorded. WD J1011+2845, a nearby white dwarf just 48 light-years from Earth, has a deep convection zone that has brought carbon from its core to the surface.
The resulting carbon molecules distort the star’s light so strongly that almost all of it is concentrated at wavelengths longer than 600 nanometers, producing a deep salmon-pink glow unlike anything seen in an ordinary star.
The Average White Dwarf Really Is White
The team also examined a representative sample of 151 white dwarfs within 20 parsecs—approximately 65 light-years—of Earth. Five showed unusual colors: two were yellow and three were pink. This suggests that approximately one in 30 white dwarfs in the Universe shines with an unusual hue.
For the remaining 146, the team measured color temperature, which compares an object’s color with that of an ideal emitter and is commonly used to describe light bulbs: “warm white” at 3,000 K and “cool white” at 6,000 K. The sample’s median color temperature was approximately 6,800 kelvins, which we perceive as white.
This corrects a common misconception in astronomy that white dwarfs are usually blue. Hotter, bluer white dwarfs are brighter and can be seen from farther away, so they dominate surveys based solely on brightness.
Professor Boris Gänsicke of the University of Warwick’s Department of Physics said: “One in 30 may not seem like a large fraction, but across the entire population of white dwarfs in the Universe, it still amounts to many strangely colored stars. However, when we take the median for a representative population of these stars, we find that the average white dwarf really is white.”
